1. Field of the Invention
The present invention relates to an apparatus for driving an electric car by inverter-controlled motors through a gear mechanism.
2. Description of the Prior Art
FIGS. 1A, 1B, and 2 show an apparatus for driving a car according to a prior art. The apparatus has a large gear 11, a small gear 12, an AC motor 13, an inverter 14, a filter reactor 15, and a feeder system 16. The large gear 11 is coaxial to and directly connected to a wheel shaft 10 and is mechanically arranged to mesh with the small gear 12. The AC motor 13 generates torque, which is amplified at an amplification factor defined by a gear ratio of the small gear 12 and large gear 11, thereby providing acceleration and deceleration force to drive the car. Also shown are a truck 9 and a smoothing capacitor 8.
The problems of the prior art will be explained. To transmit large torque, each tooth of each gear must have a size (an area) larger than a predetermined value. Mechanical strength and a limited underfloor space restrict the maximum numbers of teeth of gears to restrict a maximum gear ratio. This puts limits on a torque amplification factor of gears and the speed of a motor. Namely, the prior art restricts the speed of the motor of the car driving apparatus, and therefore, is impossible to reduce the size, weight, and cost of the apparatus by using the merit of operating the motor at high speed.
Inverters used to supply electricity to motors for driving a car usually operate on an input DC voltage of about 1500 V. This voltage is extremely high compared with an input DC voltage of about 280 V for standard inverters. As a result, the inverters for cars cause a serious harmonic induction problem. In addition, the inverters for cars are designed according to special specifications, and therefore, involve high costs.
A first object of the present invention is to provide an apparatus for driving and controlling a car, capable of increasing a gear ratio defined by small gears driven by AC motors and a large gear directly connected to a wheel shaft. The apparatus is small, light, and low-cost because it can operate the AC motors at high speed. The first object of the present invention is achieved by directly connecting a large gear to a wheel shaft, meshing the large gear with small gears, connecting the small gears to AC motors, and connecting the AC motors to AC sides of inverters, respectively. This arrangement distributes and reduces torque applied to the small gears, to decrease torque to be transmitted by each small gear. This enables the cross-sectional area of each tooth of each small gear to be reduced, thereby enabling a gear ratio defined by the gears to be increased.
A second object of the present invention is to provide an apparatus for driving a car, capable of reducing an input DC voltage for an inverter for driving an AC motor to a standard inverter level, minimizing the harmonic induction problem, and improving reliability by using low-cost, mass-produced standard inverters. The second object of the present invention is achieved by connecting small gears to AC motors, respectively, driving the AC motors by inverters, respectively, connecting the DC input sides of the inverters in series in multiple stages, and supplying power thereto from a high-voltage power source. This arrangement reduces an input DC voltage for each inverter to a standard inverter level.
More precisely, a first aspect of the present invention provides an apparatus for driving and controlling a car, having a large gear directly connected to a wheel shaft, small gears meshing with the large gear, AC motors connected to the small gears, respectively, and inverters connected to the AC motors, respectively, for supplying power thereto. The DC input sides of the inverters are connected in series. A positive terminal of one of the inverters arranged in the first stage and a negative terminal of one of the inverters arranged in the last stage are connected to a high-voltage DC power source.
This apparatus distributes torque to the small gears to drive the large gear that is directly connected to the wheel shaft. Each small gear bears small torque, and therefore, each tooth of each gear may have small mechanical strength. This means that the cross-sectional area of each tooth of each small gear may be reduced, the number of teeth of the large gear meshing with the small gears may be increased, a gear ratio defined by the small and large gears may be increased, and the size, weight, and cost of the apparatus may be reduced by operating the motors at high speed. DC input terminals of the inverters are connected in series to receive power from the high-voltage DC power source, while an input DC voltage to each inverter is dropped to a standard inverter level. This minimizes the harmonic induction problem. Since the apparatus may employ mass-produced standard inverters as they are, it is producible at low cost and high reliability.
Each of the inverters may have an inverter controller that receives a DC source voltage value detected on a power-source feeder line and an input voltage value detected at the inverter, finds a difference between the input voltage value and a quotient obtained by dividing the DC source voltage value by the number of the series-connected inverters, and corrects an output torque command for the inverter according to the difference. This arrangement suppresses a DC voltage imbalance caused by the individual differences of the series-connected inverters and prevents an input overvoltage to the inverters due to the DC voltage imbalance that may stop the apparatus.
Each of the inverters may have a short-circuit switch between the positive and negative DC input terminals thereof, to short-circuit the inverter if the inverter fails, so that the remaining inverters may continue to operate. Even if some inverters fail, this arrangement minimizes torque reduction and continues the operation of the car.
The apparatus may have a breaker for breaking the DC power to the DC input sides of the inverters, if at least one of the inverters fails so that the car must be operated with the remaining inverters, and at the same time, if the value of a DC source voltage exceeds the sum of allowable input DC voltage values of the remaining inverters. If some inverters fail so that the car must be operated with the remaining inverters, and at the same time, if a DC input overvoltage that is above the withstand voltage values of the remaining inverters is applied to the remaining inverters, the breaker opens to prevent the breakdown of the remaining inverters.
The apparatus may have an inverter controller for preparing an inverter output voltage command according to a composite value of output currents of the inverters so that the inverters may provide pulse patterns according to the same voltage command. This arrangement automatically equalizes DC voltage and continues the powering operation of the car without a special balance control of DC voltage.
A frame of each AC motor may electrically be short-circuited with a power source ground. The apparatus may have an inverter controller for providing a phase difference for a triangular wave that is used to form a PWM pulse pattern of each inverter. This arrangement minimizes a high-frequency leakage current to the ground due to the PWM switching of the inverters and maintains the quality of signals and communication.
The apparatus may have an inverter controller for each of the inverters, for calculating an output effective power value according to an input DC voltage value and an input DC current value both detected at the inverter, calculating an effective power command value according to a motor angular frequency and a torque command value related to the inverter, calculating a difference between the output effective power value and the effective power command value, and correcting the torque command value according to the difference. This arrangement suppresses an output torque imbalance, which finally causes a DC voltage imbalance, due to the individual differences of permanent-magnet flux of the motors and prevents the stoppage of the operation of the apparatus due to an inverter input over-voltage caused by the DC voltage imbalance.
A second aspect of the present invention provides an apparatus for driving and controlling a car, having a large gear provided for each of at least two wheel shafts of the car, the large gear being directly connected to the wheel shaft, small gears meshing with the large gear, AC motors connected to the small gears, respectively, inverters connected to the AC motors, respectively, to supply power thereto, DC input terminals of the inverters that drive the AC motors that are connected to the different wheel shafts are connected in series in multiple stages to a DC power source, and a breaker for breaking the DC power to all of the inverters.
If some inverters fail, and at the same time, if the driving force of the car must be maintained by increasing the current and torque of the sound inverters and motors, the apparatus of the second aspect transmits torque to rails through the two wheel shafts. This reduces the risk of wheels idling on the rails and secures acceleration for the car.
A third aspect of the present invention provides an apparatus for driving and controlling a car, having a large gear directly connected to a wheel shaft, a small gear meshing with the large gear, a 3-by-n-phase AC motor (n is an integer equal to or larger than 2) connected to the small gear, and n inverters for supplying power to the AC motor. DC input sides of the inverters are connected in series in n stages to a DC power source.
DC input terminals of the inverters are connected in series to receive power from a high-voltage feeder line. This arrangement reduces a DC input voltage to the inverters to a standard inverter level and minimizes the harmonic induction problem. This arrangement employs mass-produced standard inverters as they are to reduce costs and improve reliability.
Each of the n inverters may have an inverter controller that receives a DC source voltage value detected at the DC power source and an input voltage value detected at the inverter, finds a difference between the input voltage value and a quotient obtained by dividing the DC source voltage value by n, and corrects an output torque command according to the difference. This arrangement suppresses a DC voltage imbalance caused by the individual differences of the series-connected inverters and prevents an input overvoltage to the inverters due to the DC voltage imbalance that may stop the apparatus.
Each of the n inverters may have a short-circuit switch between the positive and negative DC input terminals thereof, to short-circuit the inverter if the inverter has trouble, so that the remaining inverters may continue to operate. Even if some inverters fail, this arrangement minimizes torque reduction and continues the operation of the car.
The AC motor may be a permanent-magnet synchronous motor, and the apparatus may have an inverter controller. If at least one of the inverters fails, the inverter controller controls the remaining sound inverters to pass a current in a direction to cancel permanent-magnet flux of the motor. The permanent-magnet synchronous motor has a high inductive voltage to improve efficiency and reduce the size of the apparatus.
A fourth aspect of the present invention provides an apparatus for driving and controlling a car, having a large gear directly connected to a wheel shaft, small gears meshing with the large gear, AC motors connected to the small gears, respectively, inverters connected to the AC motors, respectively, for supplying power thereto, converters connected to the DC input sides of the inverters, respectively, having AC input terminals connected in series in multiple stages, and a transformer for receiving power from an AC power source and providing a proper rated AC voltage to the converters.
According to the fourth aspect, the AC input terminals of the inverters are connected in series through the converters, to receive power from a high-voltage feeder line, so that a DC input voltage to the inverters is dropped to a standard inverter level. This minimizes the harmonic induction problem. The inverters of the fourth aspect may be mass-produced standard inverters to reduce costs and improve reliability. The fourth aspect multiplexes the converters in series to apply multiple PWM voltage levels to a secondary winding of the transformer. This reduces harmonics escaping to the power source system, as well as heat loss and noise caused on the transformer due to the harmonics.
Each of the converters may have a short-circuit switch between the AC input terminals of the converter. The short circuit switch short-circuits the converter if the converter or the inverter connected to the DC output side of the converter fails, so that the remaining converters and inverters may continue to operate. Even if some of the inverters or converters fail, this arrangement minimizes a torque reduction and continues the operation of the car.
A fifth aspect of the present invention provides an apparatus for driving and controlling a car, having a large gear directly connected to a wheel shaft, small gears meshing with the large gear, AC motors connected to the small gears, respectively, inverters whose AC output sides are connected to the AC motors, respectively, to supply power thereto, the DC input sides of the inverters being connected in series, a transformer for receiving power from an AC power source and providing a proper rated AC voltage, a converter whose DC output side is connected to a positive terminal of one of the inverters arranged in the first stage and a negative terminal of one of the inverters arranged in the last stage and whose AC input side is connected to the secondary side of the transformer, and short-circuit switches connected to the inverters, respectively, each between the positive and negative input terminals of the corresponding inverter. Each of the short-circuit switch is closed if the corresponding inverter fails, so that the remaining inverters may continue to operate.
The fifth aspect connects the DC input terminals of the inverters in series to receive power from a high-voltage feeder line through the common converter. This decreases a DC input voltage to the inverters to a standard inverter level and minimizes the harmonic induction problem. The inverters of the fifth aspect may be mass-produced standard inverters to reduce costs and improve reliability. In addition, the fifth aspect arranges the short-circuit switch between the positive and negative DC input terminals of each inverter. Even if some inverters fail, this arrangement minimizes torque reduction and continuously operates the car.
The apparatus for driving and controlling a car according to any one of the first to fifth aspects of the present invention is capable of mechanically integrating an AC motor and an inverter into a unit and operating the AC motor at high speed to spare a space in a truck. This space is used to accommodate the motor-inverter unit. This eliminates wiring between the motor and the inverter. Further, this eliminates an underfloor space to be prepared according to the prior art for accommodating an inverter. This enables even a motor car to be constructed into a double-decker structure to accommodate more passenger seats.